Although the invention is believed to have applicability for a wide variety of fluid handling systems, primary experience to date has been in the context of well casings. Therefore, the following description will be in that environment.
For many years well casings have been constructed of metal, usually steel. Metal casing systems have several disadvantages. Fittings for securing casing sections together are usually welded on the adjoining end of the pipe sections as an on-site procedure, resulting in the need for several days construction time for most wells. The attendant costs of this procedure are evident. In addition, there is an increased risk of collapse of the uncased parts of the well.
Further, metal casings are subject to corrosion resulting from chemical and electrolytic attack that shorten the life of the casing. It is also believed that these factors render metal casings, even stainless steel parts such as well screens, less resistant to the development of colonies of bacteria that are sometimes present in water wells.
In order to overcome the foregoing disadvantages, the use of casings of synthetic polymeric materials, for example, polyvinyl chloride (PVC), have been proposed. They primarily involve the use of PVC pipe, each section having a bell end into which the leading end of the next successive pipe end is solvent welded. While such casing systems alleviate the problems mentioned above regarding metal casings, there are still construction delays because a significant period of time is necessary for the solvent weld to cure to a sufficient extent so that it will withstand the pull from the weight of the casing already placed in the well.
With respect to well screens, prior designs have been formed from standard plastic pipe sections in which transverse slits are cut by saw blades. These slits are straight-sided and can become clogged with sand particles that wedge themselves in the slits. Further, the slits are cut from the outside of the pipe inwardly by circular blades, and thus the flow cross-section area of the screen per unit length is restricted, because of the material that must be left at the ends of each slit by the blade.
One attempt to overcome these drawbacks is a metal well screen comprising keystone wire wound on longitudinally extending support rods, as shown in U.S. Pat. No. 3,816,894. Similar well screens of plastic materials, in which a plastic wire is wound on an extruded core having raised sections are shown in U.S. Pat. Nos. 3,378,420 and 4,068,713. While these constructions achieve an efficient screen design, the wire is very easily knocked loose from the support rods or cores when the screen is lowered into the well or when the screen is cleaned, for example, by rotational jet cleaning that directs a high velocity jet of cleaning fluid outwardly from the interior of the screen. In addition, manufacturing costs are quite high.